1. Field of the Invention
This invention relates to a liquid crystal display device, specifically to a liquid crystal display device in which an alignment direction of liquid crystal molecules is controlled by a lateral electric field generated between a pixel electrode and a common electrode.
2. Description of the Related Art
As one way of achieving a wide viewing angle of the liquid crystal display device, a method to realize a light switching function by rotating the liquid crystal molecules in a plane parallel to the substrate with the lateral electric field generated between the electrodes on the same substrate has been developed. In-Plane Switching (hereafter referred to as IPS) method and Fringe-Field Switching (hereafter referred to as FFS) method are known as examples of technologies mentioned above.
A manufacturing process of the liquid crystal display device according to the FFS method will be explained referring to the drawings. FIGS. 17A through 19B show a manufacturing process of one pixel in the liquid crystal display device according to the FFS method. FIGS. 17A, 18A and 19A are plan views. Each of FIGS. 17B, 18B and 19B is a cross-sectional view showing a section X-X in each of the FIGS. 17A, 18A and 19A, respectively. Although a plurality of pixels is disposed in a matrix form in the actual liquid crystal display device, only one pixel is shown in each of the drawings.
A buffer layer 11, which is made of a silicon dioxide (SiO2) film or a silicon nitride (SiNx) film, and an amorphous silicon layer are successively formed by CVD (Chemical Vapor Deposition) on a TFT substrate 10, which is made of a glass substrate or the like, as shown in FIG. 17B. The amorphous silicon layer is crystallized by excimer laser annealing and transformed into a polysilicon layer. The polysilicon layer is patterned to form a U-shaped active layer 12 of a thin film transistor 1.
After that, a gate insulation film 13 is formed over the active layer 12. A gate line 14 made of chromium, molybdenum or the like is formed on the gate insulation film 13 overlapping the active layer 12. The gate line 14 extends in a row direction, and intersects the active layer 12 at two locations. A gate signal that controls turning on/off of the thin film transistor 1 is applied to the gate line 14. On the other hand, a common electric potential line 15, that is made of the same material as the gate line 14 and is for providing a common electric potential Vcom, is formed parallel to the gate line 14.
Next, there is formed an interlayer insulation film 16 that covers the thin film transistor 1 and the common electric potential line 15. And contact holes CH1 and CH2, that expose a source region 12s and a drain region 12d in the active layer 12 respectively, are formed in the interlayer insulation film 16. Also, a contact hole CH3, that exposes the common electric potential line 15, is formed in the interlayer insulation film 16.
There are formed a source electrode 17 that is connected with the source region 12s through the contact hole CH1, a display signal line 18 that is connected with the drain region 12d through the contact hole CH2, and an electrode 19 that is connected with the common electric potential line 15 through the contact hole CH3. The source electrode 17, the display signal line 18 and the electrode 19 are made of a metal such as aluminum, an aluminum alloy or the like. Next, a planarization film 20 is formed over the entire surface. Contact holes CH4 and CH5, that expose the source electrode 17 and the electrode 19 respectively, are formed in the planarization film 20.
And there is formed a pixel electrode 21 that is connected with the source electrode 17 through the contact hole CH4 and extends over the planarization film 20, as shown in FIG. 18B. The pixel electrode 21 is made of a first layer transparent electrode such as ITO (Indium Tin Oxide), and is applied a display signal Vsig from the display signal line 18 through the thin film transistor 1.
After that, an insulation film 22 is formed to cover the pixel electrode 21, as shown in FIG. 19B. A contact hole CH6, that exposes the electrode 19, is formed by etching the insulation film 22. A common electrode 23, that has a plurality of slits S, is formed on the pixel electrode 21 through the insulation film 22. The common electrode 23 is made of a second layer transparent electrode such as ITO, and is connected with the electrode 19 through the contact hole CH6.
A counter substrate 30 made of a glass substrate or the like is disposed facing the TFT substrate 10. A polarizing plate 31 is attached to the counter substrate 30. Also, a polarizing plate 32 is attached to a back surface of the TFT substrate 10. The polarizing plates 31 and 32 are disposed in a way that their polarization axes are perpendicular to each other. A liquid crystal 40 is sealed-in between the TFT substrate 10 and the counter substrate 30.
In the liquid crystal display device described above, an average alignment direction (hereafter simply referred to as “alignment direction”) of major axes of the liquid crystal molecules of the liquid crystal 40 is perpendicular to the polarization axis of the polarizing plate 32 when a display voltage is not applied to the pixel electrode 21 (no voltage state). In this case, linearly polarized light passing through the liquid crystal 40 does not go through the polarizing plate 31 because its polarization axis is perpendicular to the polarization axis of the polarizing plate 31. That is, black is displayed (normally black).
When the display voltage is applied to the pixel electrode 21, on the other hand, there is caused an electric field from the pixel electrode 21 toward the underlying common electrode 23 through the slits S. The electric field is perpendicular to a longitudinal direction of the slits S on the plan view, and the liquid crystal molecules are aligned parallel to or perpendicular to a line of electric force of the electric field. At that time, the linearly polarized incident light to the liquid crystal 40 is turned into elliptically polarized light by birefringence to have a component of linearly polarized light that passes through the polarizing plate 31. In this case, white is displayed. The liquid crystal display device according to the FFS method is disclosed in Japanese Patent Application Publication No. 2002-296611.
In the liquid crystal display device, a voltage applied to the liquid crystal is inverted once every frame period by inverting a polarity relative to the common electric potential Vcom of the display signal Vsig applied to the display signal line 18 in order to prevent deterioration of the liquid crystal. However, with a frame inversion drive, which is a method to invert all of an image area at a time, quality of the display is worsened by a flicker or the like. Thus, a line inversion drive in which the inversion of the polarity is performed by each horizontal line, and a dot inversion drive in which the inversion of the polarity is performed by each dot have been known as effective methods to suppress the flicker. In addition, a common electrode AC drive, which is a method to provide the common electrode with alternating voltage, is known as a method effective to reduce a voltage to drive a driver IC and a circuit incorporating the TFTs.
When the line inversion drive and the common electrode AC drive are combined together in a conventional liquid crystal display device, there has been a problem that unevenness in brightness due to a signal distortion is apt to be caused to reduce the quality of the display, because the voltage at the common electrode is inverted every horizontal period.
In addition, reducing the drive voltage has been difficult because it is not possible to combine the dot inversion drive with the common electrode AC drive in the conventional liquid crystal display device. Furthermore, because the dot inversion drive requires a circuit to dot-invert the polarity of the display signal Vsig, there has been a problem that a structure of a peripheral circuit of the pixel becomes more complicated as well as increased power consumption due to the inversion drive.